1. Technical Field of the Invention
The present invention relates to telecommunication networks. More particularly, and not by way of any limitation, the present invention relates to a system and method for interconnecting a network domain carrying Signaling System No. 7 (SS7) traffic in one type with another network domain operable to carry SS7 traffic of a different type.
2. Description of Related Art
Out-of-band signaling establishes a separate channel for the exchange of signaling information between call component nodes in order to set up, maintain and service a call in a telephony network. Such channels, called signaling links, are used to carry all the necessary signaling messages between the nodes. Thus, for example, when a call is placed, the dialed digits, trunk selected, and other pertinent information are sent between network switches using their signaling links, rather than the trunks which will ultimately carry the bearer traffic, i.e., conversation.
Out-of-band signaling has several advantages that make it more desirable than traditional in-band signaling. First, it allows for the transport of more data at higher speeds than multi-frequency (MF) outpulsing used in the telephony networks without it. Also, because of separate trunks and links, signaling can be done at any time in the entire duration of the call, not just at the beginning. Furthermore, out-of-band signaling enables signaling to network elements to which there is no direct trunk connection.
Signaling System No. 7 (SS7) provides a packet-based signaling architecture that has become the out-of-band signaling scheme of choice between telephony networks and between network elements worldwide. Three essential components are defined in a signaling network based on SS7 architecture. Signal Switching Points (SSPs) are basically telephone switches equipped with SS7-capable software that terminate signaling links. SSPs generally originate, terminate, or switch calls. Signal Transfer Points (STPs) are the packet switches of the SS7 network. In addition to certain specialized functions, they receive and route incoming signaling messages towards their proper destination. Finally, Service Control Points (SCPs) are databases that provide information necessary for advanced call-processing and Service Logic execution.
As is well known, SS7 signaling architecture, effectuated as a multi-layered protocol over a Time Division Multiplex (TDM) transport, is standardized under the American National Standards Institute (ANSI) and the International Telecommunications Union (ITU) to operate as the common “glue” that binds the ubiquitous autonomous networks together so as to provide a “one network” feel that telephone subscribers have come to expect. Furthermore, SS7 signaling has made it possible to provision a host of advanced services (or, Value-added Services) based on Intelligent Network (IN)/Advanced Intelligent Network (AIN) architectures in both wireless and wireline telecommunications networks.
Due to the phenomenal growth in popularity of the Internet, there has been a tremendous interest in using packet-switched network (PSN) infrastructures (e.g., those based on Internet Protocol (IP) addressing) as a replacement for, or as an adjunct to, the existing circuit-switched network (CSN) infrastructures used in today's telephony. From the network operators' perspective, the inherent traffic aggregation in packet-switched infrastructures allows for a reduction in the cost of transmission and the infrastructure cost per end-user. Ultimately, such cost reductions enable the network operators to pass on the concomitant cost savings to the end-users.
Additional factors that are driving the current trend in transporting the bearer traffic on integrated and/or hybrid networks are: improvements in the quality of Voice-over-IP (VoIP) telephony; the Internet phenomenon; emergence of standards; cost-effective price-points for advanced services via media-rich call management, et cetera. Some of the emerging standards in this area are the well known H.323 protocol, developed by the ITU, Session Initiation Protocol (SIP) or Internet Protocol Device Control (IPDC) by the Internet Engineering Task Force (IETF), or Simple/Media Gateway Control Protocol (SGCP or MGCP). Using these IP-based standards, devices such as personal computers can inter-operate seamlessly in a vast inter-network, sharing a mixture of audio, video, and data across all forms of packet-based networks which may interface with circuit-switched network portions.
To seamlessly integrate carrier-grade service architectures within IP-based networks, it has therefore become necessary to provide the capability to transport out-of-band signaling information (such as the SS7 signaling) on IP connections also. In a well-known arrangement, for example, such SS7-over-IP transport utilizes a connection-oriented IP transport protocol, called Stream Control Transmission Protocol (SCTP), for transmitting SS7 signaling messages across the network elements. Clearly, it is highly desirable that such transport not disrupt or degrade the capabilities of the signaling network, as they are essential in effectuating various advanced services. In particular, it is necessary for applications involving the higher layers of the SS7 protocol (e.g., Transaction Capabilities Application Part or TCAP, Signaling Connection Control Part or SCCP, or various User Parts such as ISDN User Part or ISUP, Telephony User Part or TUP, and Data User Part or DUP) to operate without any degradation when the SS7 messages are transported by means of SCTP. That is, message dialogs (e.g., call setup, etc.) in these applications should remain unaffected even when the messages are sent over IP (transport-independency). Accordingly, SS7-over-IP mechanisms are required to satisfy the following requirements which are traditionally provisioned in pure SS7 networks:                High reliability;        High availability;        Short error handling time; and        Extremely low error rates.        
In general, the functionality of the lower Message Transfer Part (MTP) portion of the SS7 protocol (Level-2 MTP (MTP2) and Level-3 MTP (MTP3) layers, in particular) is responsible for link control and management, network reliability, error handling, etc. Consequently, the MTP functionality of the messages must be preserved as much as possible as they are transported over SCTP.
Based on the foregoing discussion, it should be evident that traditional SS7 networks (using TDM as transport mechanism) are being required to coexist with networks having non-traditional SS7 links, e.g., SS7-over-IP links. Accordingly, there is a need to provide seamless functionality to interconnect an SS7 link operating in one domain with another SS7 link in a different domain. Traditional solutions for coupling two SS7 links typically have typically required the use of MTP3 routing layer, thereby necessitating highly complex implementation.